The Back Frame
The back frame is made of large wooden posts which support the cast-iron frame. These strong braces fit snugly inside the rim (the picture at the right shows the back frame pulled slightly away from the rim), and prevent the piano from twisting and warping under the high tension of the strings. The braces are connected tightly by the use of dovetail joints, which are the strongest wood joints possible. The cast-iron frame is bolted to the back frame support beams, which are usually made of spruce - a wood that is known for its resistance to splitting and cracking. The tone collector is the metal plate that unifies the spruce braces into a common vibrating area which enhances the ability of the piano to project its sound. Also seen in the picture at the right is the keybed, the part of the piano's inner structure which supports the keyframe and keyboard.